Prior art devices have been used to detect the exact location of a moving object for purposes of computing the velocity of the object. Devices for accurately computing the velocity of a moving object have various applications in material testing, product testing, and other testing wherein products are subject to impact. Impact testing machines generally employ a weight or mass that falls onto a subject. The subject may deform, break, or fracture, or in some other way absorb energy from the falling weight. Various characteristics of the subject may be determined, such as fracture resistance and energy absorption of the subject material. Such subject materials may be samples of metals, plastics, or safety helmets.
These characteristics are usually obtained by drop or pendulum tests where it is necessary to measure the velocity of the moving object just prior to impact and/or just after impact. The velocity measurements are used to derive energy input, loss, absorption, and materials restitution. For example, computations of velocity may be used to compute the impact energy in accordance with the following equation: EQU E.sub.i = 1/2 mV.sup.2
where:
E.sub.i = Energy input PA1 m = mass of object PA1 V = velocity of object PA1 E.sub.o = Energy loss PA1 m = mass of object PA1 V.sub.1 = preimpact velocity PA1 V.sub.2 = rebound of postimpact velocity PA1 W.sub.D = displacement between the two precisely determined locations within the light beam. PA1 .DELTA. t = time displacement between the point of time at which the forward edge of the object intersects the first precisely determined location within the light beam and the point of time at which the trailing edge of the object intersects the second precisely determined location.
The energy loss may be computed in accordance with the following equation: EQU E.sub.o = 1/2 m (V.sub.2.sup.2 - V.sub.1.sup.2)
where:
The coefficient of restitution of elastic materials may be obtained from the following equation: ##EQU1##
Various prior art devices for measuring the velocity of a moving object have been generally too expensive and/or inaccurate for simple laboratory tests. Several of these prior art devices have sought to provide a measurement of the velocity of a moving object by measuring the duration of time elapsed between the intersection of two light beams. Such devices typically include means for sensing the intersection of the light beams by the moving object, which means are coupled to analog circuitry for computing the elapsed time. The device may be pre-programmed with data with respect to the displacement between the two beams and to thereby compute the velocity of the moving object.
Such devices are typically inaccurate since they typically fail to provide intersection of the light beams to thereby activate the associated circuitry, at precisely defined locations within the respective light beams. Furthermore, the associated analog circuitry typically used to perform an integration function is sensitive to temperature and component tolerance shifts. As a result, these prior art devices fail to provide measurements of velocity which may be used to provide data which meets the required degree of accuracy.